1. Field of the Invention
The present invention relates to a liquid ejection apparatus and a liquid ejection head.
2. Description of the Related Art
A recording system that uses a liquid ejection apparatus having a liquid ejection head configured to eject a liquid, such as ink, has various applications not only recording images on a recording material, such as paper. Hereinafter, “recording” refers to forming a desired dot group by ejecting a liquid including applications other than recording images on a recording material. The smallest component of the dot group to be formed is referred to as a pixel. The entire mechanism for ejecting a liquid provided in a liquid ejection head and consisting of ejection ports for ejecting droplets, pressure chambers communicating with the ejection ports, flow channels for supplying the liquid to the pressure chambers, and the like is referred to as an ejection mechanism unit.
There have recently been demands for high definition recording. To meet these demands, liquid ejection heads having a large number of ejection mechanism units arranged with high density have been developed. Arranging the ejection mechanism units with high density can be achieved by reducing the size of each ejection mechanism unit. Since there is a limit to the degree of reduction in size of each ejection mechanism unit, however, a large number of ejection mechanism units may be arranged in a predetermined range when the ejection mechanism units are arranged two-dimensionally.
An ordinary liquid ejection apparatus performs recording by ejecting a liquid while a recording material and a liquid ejection head are moving relatively. In a configuration in which the ejection mechanism units are arranged two-dimensionally, the size of the liquid ejection head is desirably reduced in the direction of the relative movement of the recording material.
Thus, the size of the entire liquid ejection head is reduced. Moreover, the liquid ejection head is generally manufactured at lower cost. Especially the size of a liquid ejection apparatus in which multiple liquid ejection heads are arranged, such as a color printer, is reduced. When recording is performed continuously by a plurality of liquid ejection heads, since a time difference between landing of the first droplet and landing of a subsequent droplet on a recording material, impairment in recording image quality caused by, for example, variation in bleeding of the liquid is reduced. Moreover, a decrease in relative position accuracy of dots related to conveying precision of the recording material is reduced.
To reduce the size of the liquid ejection head in the direction of the relative movement of the recording material, the ejection mechanism units need to be arranged with high density. If the size of the pixels to be recorded is constant, the range in which the ejection mechanism units are arranged in the direction of the relative movement substantially corresponds to the number of pixels per unit length/the number of ejection mechanism units per unit area. Thus, increasing the number of the ejection mechanism units per unit area is required. Moreover, the ejection ports of each ejection mechanism unit must be arranged at a pitch of the predetermined pixel in the direction perpendicular to the direction of the relative movement of the recording material.
Japanese Patent Laid-Open No. 6-155734 discloses a method for arranging ejection ports at the pitch equal to that of the pixel in the direction perpendicular to the direction of relative movement while disposing nozzles with high density, by combining four kinds of nozzles (which correspond to the ejection mechanism units).
Japanese Patent Laid-Open No. 2007-90520 discloses a method for arranging ejectors (which correspond to the ejection mechanism units) of the same configuration rotated by 180 degrees and two-dimensionally.
In the method disclosed in Japanese Patent Laid-Open No. 6-155734, the size of the liquid ejection head is reduced by combining nozzles of different types to increase the degree of freedom of arrangement and aspect ratio of the pressure chambers. Nozzles of different shapes, however, have different ejection characteristics. In the method disclosed in Japanese Patent Laid-Open No. 6-155734, there is a problem of unevenness caused in a recorded image due to the difference in the ejection characteristics of different types of nozzles.
In the method disclosed in Japanese Patent Laid-Open No. 2007-90520, as illustrated in FIGS. 10 and 11 thereof, the ejection ports are arranged at the pitch equal to that of the pixel in the direction perpendicular to the direction of the relative movement of the recording material. Details of arrangement of the ejectors, however, are not described in Japanese Patent Laid-Open No. 2007-90520. In the method disclosed in Japanese Patent Laid-Open No. 2007-90520, the ejectors are arranged at complicated distances in a manner such that the ejection ports are disposed at the pitch equal to that of the pixel. Thus, in the method disclosed in Japanese Patent Laid-Open No. 2007-90520, the entire ejection mechanism units are not necessarily arranged with high density.
FIG. 11 of Japanese Patent Laid-Open No. 2007-90520 illustrates a part of a configuration of the liquid ejection head illustrated in FIG. 10 of Japanese Patent Laid-Open No. 2007-90520. In FIG. 11 of Japanese Patent Laid-Open No. 2007-90520, the ejection ports are arranged on inclined lines between parallel main lines 6A and 6B disposed in each of the upper and lower areas of the drawing. The number of the ejection ports (i.e., ejectors) arranged on the inclined lines is recurrently four and three, and is not constant. Thus, because the ejection ports are disposed at the pitch of the pixel in the direction perpendicular to the direction of the relative movement, only three ejectors are disposed in some locations where four ejectors can be disposed actually in the up-down direction. In the method disclosed in Japanese Patent Laid-Open No. 2007-90520, the ejection mechanism units cannot be arranged with density as high as possible.
In FIG. 11 of Japanese Patent Laid-Open No. 2007-90520, the distance between ejection ports adjoining in the direction perpendicular to the direction of the relative movement of the recording material corresponds to two pixels where the adjoining ejection ports are close to each other, and 24 pixels where they are apart from each other. In Japanese Patent Laid-Open No. 2007-90520, recording is performed at a 2-pixel pitch by the ejectors of each of the upper and the lower areas of FIG. 11 and, as a whole, recording at a 1-pixel pitch is performed.
Setting the distance between adjoining ejection ports to correspond to two pixels is possible in a case where pixel density is very low, but is problematic in a case where the pitch of the pixels is small. For example, in a case where recording is performed at 1200 dots per inch (dpi), the pitch of the pixels is about 21 μm. In an ordinary design, the magnitude of an ejection port is about 20 μm. Thus, there is a problem that the thickness of the partition walls of the pressure chambers of adjoining ejectors becomes as thin as about 20 μm, of which insufficient rigidity may cause crosstalking. Moreover, if an appropriate distance is not provided between the ejection port and the partition wall, an ejecting direction of the droplet may be changed due to the existence of the partition wall.
As described above, in the methods disclosed in Japanese Patent Laid-Open Nos. 6-155734 and 2007-90520, it is difficult to arrange the ejection mechanism units with high density, while satisfying indispensable design requirements. The design requirements herein are: arranging the ejection ports at a pitch substantially equal to that of the pixel in the direction perpendicular to the direction of the relative movement of the recording material; and not impairing performance of each ejection mechanism unit even if the pitch of the pixel is small.
The present invention provides a liquid ejection apparatus and a liquid ejection head capable of arranging ejection mechanism units with high density, while satisfying indispensable design requirements.